Redmine

when I try create new Backlog in scrum plugin I get this error.

Started GET "/scrum/511/new_pbi/6?top=true" for *.*.*.* at 2019-04-09 16:48:45 +0400
Processing by ScrumController#new_pbi as JS
  Parameters: {"top"=>"true", "sprint_id"=>"511", "tracker_id"=>"6"}
  Current user: admin (id=1)
  Rendered plugins/scrum/app/views/scrum/_issue_attributes.js.erb (0.3ms)
  Rendered plugins/scrum/app/views/scrum/_issue_attributes.html.erb (65.1ms)
  Rendered plugins/scrum/app/views/scrum/_new_issue_attributes.html.erb (66.5ms)
  Rendered plugins/scrum/app/views/scrum/_new_pbi_modal.html.erb (71.1ms)
  Rendered plugins/scrum/app/views/scrum/new_pbi.js.erb (72.5ms)
Completed 200 OK in 663ms (Views: 58.5ms | ActiveRecord: 567.2ms)
Started POST "/scrum/511/create_pbi" for 172.17.10.21 at 2019-04-09 16:48:49 +0400
Processing by ScrumController#create_pbi as JS
  Parameters: {"utf8"=>"✓", "back_url"=>"https://172.17.1.185/product_backlog/511", "issue"=>{"tracker_id"=>"6", "sprint_id"=>"511", "subject"=>"saaa", "status_id"=>"1", "priority_id"=>"23"}, "top"=>"true", "create_and_continue"=>"Create and continue", "sprint_id"=>"511"}
  Current user: admin (id=1)
+*Exception: #<NoMethodError: undefined method `<' for nil:NilClass>*+
  Rendered plugins/scrum/app/views/scrum/create_pbi.js.erb (0.6ms)
Completed 200 OK in 3190ms (Views: 2.9ms | ActiveRecord: 1014.1ms)

Can anyone help find the cause and fix it?